1. Field of the Invention
This invention relates to catalytic cracking of petroleum fractions. More particularly, this invention relates to an improved process of converting easily coked petroleum fractions into coke and valuable hydrocarbon products, such as gas, gasoline, light cycle gas oil and heavy cycle gas oil in a fluid catalytic cracking reactor.
2. Description of Prior Art
Conversion of various petroleum fractions to more valuable products in catalytic reactors is well known in the art. The petroleum industry has found the use of a fluid bed catalytic cracker reactor (hereinafter FCC reactor) particularly advantageous for that purpose. An FCC reactor typically comprises a thermally balanced assembly of apparatus comprising a reactor vessel filled with a catalyst and a regenerator vessel wherein spent catalyst is regenerated. The feed is converted in the reactor vessel over the catalyst, and coke simultaneously forms on the catalyst, thereby deactivating the same. The deactivated (spent) catalyst is removed from the reactor vessel and conducted to the regenerator vessel, wherein coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. The reactor-regenerator vessel assembly must be maintained in steady state heat balance so that heat generated by burning the coke provides sufficient thermal energy for catalytic cracking in the reactor vessel. The steady-state heat balance is usually achieved and maintained in the FCC reactors by controlling the rate of flow of the regenerated catalyst from the regenerator to the reactor. The rate of catalyst flow is normally controlled by means of a slide valve in the regenerator-to-reactor conduit. The degree of opening of the slide valve is controlled by a conventional controlling means coupled to a temperature sensing means (e.g., a thermocouple), placed at the outlet of the reactor, to maintain the desired temperature inside the reactor.
The product stream of the catalytic cracker is usually fractionated into a series of products, including: gas, normally conducted to gas concentration plant; gasoline; light cycle gas oil; and heavy cycle gas oil. A portion of the heavy cycle gas oil is usually recycled into the reactor vessel and mixed with fresh feed. The bottom effluent of the fractionator is conventionally subjected to settling and the solid portion of the settled product is also recycled to the reactor vessel in admixture with the heavy cycle gas oil and feed.
In a modern version of fluid catalytic cracking reactor, the regenerated catalyst is introduced into the base of a riser column in the reactor vessel. The riser column or riser serves a two-fold purpose: (1) to transfer the catalyst from the regenerator to the reactor, and (2) to initiate cracking of the petroleum feed. The regenerated hot catalyst is admixed in the riser inlet or upstream section of the riser (i.e., in the bottom of the riser column if the riser column is positioned substantially vertically and the flow of the feed and the catalyst is in the upward direction) with a stream of fresh feed and recycled petroleum fractions, and the mixture is forced through the riser column. During the passage of the catalyst and of the petroleum fractions through the riser the petroleum is cracked, and coke is simultaneously deposited on the catalyst. The fluid bed of the coked catalyst and of the cracked and reformed petroleum components is passed upwardly out of the riser and through a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor. The cracked petroleum fraction is conducted to product separation, while the coked catalyst passes to the regenerator vessel, and is regenerated therein, as discussed above.
It has also been proposed to position the riser on top of the reactor vessel in such a manner that the regenerated catalyst mixed with the FCC feedstock is forced to flow downwardly (see U.S. copending application of Gross et al., Ser. No. 254,329, filed Apr. 14, 1981, the entire contents of which is incorporated herein by reference). As set forth in that copending application, the downflow configuration of the riser reactor unexpectedly improves conversion-coke and gasoline selectivity of the FCC process and increases octane rating of the gasoline produced in the process.
Further details of FCC processes can be found in U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210 (Leffer); 3,338,821 (Moyer et al); 3,812,029 (Snyder, Jr.); 4,093,537 (Gross et al); and 4,118,338 (Gross et al); as well as in Venuto et al, Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekher, Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference.
FCC reactions are endothermic in nature with the highest temperatures of about 1000.degree. to 1050.degree. F. observed at the inlet of the riser and continually falling along the reaction path. The lowest outlet temperature at the riser outlet and at the top of the reactor must usually be maintained below certain limits, e.g., below about 1040.degree. F., because of limitations in heat transfer capacity of the downstream distillation columns. In addition, excessive temperature may cause maintenance problems, such as undue riser expansion and mechanical stress on the expansion joints. Conversely, temperatures in the upstream and downstream parts or sections of the riser often reach levels much higher than those at the outlet of the riser. The term "downstream part or section of the riser" is defined herein as the riser section intermediate the inlet and the outlet of the riser, the latter being the section where the suspension of catalyst, products and unconverted feed leaves the riser. Excessive temperatures are controlled by decreasing catalyst circulation rate, which lowers the mix inlet temperature. However, decreased catalyst circulation rate may lead to more complete combustion of the coke on the catalyst. Less coke left on the regenerated catalyst increases catalyst activity. This activity increase leads to more coke yield and higher regenerator temperatures, causing the rate of the catalyst circulation to decrease even further, or, as is commonly referred to in the art, to "wind-down". This condition must be noted quickly and appropriate steps must be taken to restabilize the heat balanced operation, wherein the heat generated by regeneration of the catalyst in the regenerator does not exceed the temperature limits of optimum operation in the riser. Removal of the heat from the downstream portion of the riser to a heat sink (cooler) outside of the system, thereby uncoupling the heat balance, is inefficient because valuable heat energy is removed from the process.
It is a primary object of this invention to increase the yield of and the octane number of gasoline produced in the FCC plant containing downflow reactor riser within the temperature constraints imposed on the reactor by downstream heat removal limitations.
It is an additional object of this invention to increase temperature levels in the regenerator portion of such an FCC plant without upsetting overall heat balance of the process.
It is another object of this invention to provide an improved method of catalyst regeneration in an FCC plant with the downflow reactor riser.
It is yet another object of this invention to provide an improved FCC reactor/regenerator assembly apparatus wherein at least a portion of the downflow reactor riser is equipped with a heat exchanging means.
Additional objects and advantages of this invention will become apparent to those skilled in the art from the study of this specification and of the appended claims.